The present invention relates to a driving force control unit for a vehicle which is capable of switching the driving force to be transmitted to the driving wheels based on the brake pedal depression.
In the prior art, vehicles exist in which the driving force is transmitted to the driving wheels when the transmission is set in a driving range and the vehicle is idling at a certain or lower vehicle speed, including when the vehicles stop. The driving force in an idling condition is called xe2x80x9ccreep forcexe2x80x9d, by which unintentional backward displacement of the vehicle on a slope is restricted or the running operation in traffic is improved. In this conventional vehicle, creep force is generated even when the brake pedal is depressed and the vehicle is in an idling condition at the certain or lower vehicle speed. At this time, the creep force generated by the rotation of the engine is forcibly restricted by the braking force, which can cause vehicle vibration or noise.
In order to solve this problem, Japanese Patent Publication No. 1-244930 (i.e., Japanese Patent Application No.63-71520) discloses a control unit for a vehicular automatic clutch which is applied to a control system for establishing a low dragging torque (creep force) when the transmission is set in a driving range and the vehicle moves at an extremely low speed. The control unit makes the creep force lower when the brake pedal is depressed than when the brake pedal is released. According to this disclosure, the creep force is changed by depressing the brake pedal from a high state (strong creep condition) to a low state (weak creep condition) so that the aforementioned problems, such as vibrations when the vehicle stops, can be eliminated.
In Japanese Patent Publication No.9-202159 (i.e., Japanese Patent Application No. 8-12457), there is also disclosed a vehicle with a starting clutch, in which the starting clutch is half engaged to apply driving force (creep force) to the vehicle when the transmission is set in a driving range and the vehicle moves at an extremely low speed. In this vehicle, the engaging state of the starting clutch is controlled in response to a braking operation of the driver so that the driving force is made lower (in the weak creep condition) when the brake pedal is depressed than when the brake pedal is released.
However, if the brake pedal depression is released while the vehicle stops, the driving force is switched from the low state to the high state. When the driving force value in the high state upon switching from the low state is set at a smaller value, the vehicle can start smoothly on a flat road, but is likely to displace backwards on an up slope when the accelerator pedal is depressed. On the other hand, when the aforementioned driving force value in the high state is set at a larger value, the vehicle can start smoothly on an up slope without any backward displacement, but may start abruptly on a flat road. As to the increasing rate of the driving force upon switching from the low state to the high state when the vehicle starts, if the increasing rate is set to a higher value, the vehicle can start smoothly without any backward displacement on an up slope, but is likely to cause an abrupt start on a flat road.
In view of the foregoing problems, an objective of the present invention is to provide a driving force control unit for a vehicle which ensures that the vehicle starts smoothly.
In order to achieve this object, according to a first aspect of the invention, a driving force control unit for a vehicle is provided which allows transmission of the driving force from a driving motor to driving wheels, irrespective of the releasing of an accelerator pedal at a certain or lower vehicle speed when a transmission is set in a driving range, and which switches the magnitude of the driving force while the accelerator pedal is released at a vehicle speed of no more than the certain vehicle speed between a greater condition and a smaller condition based on the brake pedal depression so that the driving force is made lower when the brake pedal is depressed rather than when the brake pedal is released, wherein the driving force value in the greater condition upon switching from the smaller condition to the greater condition, when the brake pedal is released when the vehicle stops, is increased or decreased based on a vehicle inclination angle when the vehicle is stopping.
According to the invention, the driving force value in the greater condition after the brake pedal is released is made larger at a large vehicle inclination angle and smaller at a small vehicle inclination angle.
The vehicle inclination angle means the surface gradient of a road on which the vehicle stops. The larger the vehicle inclination angle, the steeper the surface gradient of the road so that greater driving force is required to start the vehicle.
The term xe2x80x9ca certain vehicle speedxe2x80x9d indicates a vehicle speed just before the vehicle stops. Therefore, if a certain vehicle speed is 5 km/h, as an example shown in the preferred embodiments, the vehicle speed range xe2x80x9cat a certain or lower vehicle speedxe2x80x9d includes vehicle speeds from 0 km/h (when the vehicle stops) to 5 km/h.
Meanwhile, the xe2x80x9csmaller conditionxe2x80x9d or xe2x80x9cloweringxe2x80x9d of the driving force includes the reduction of the driving force to zero (i.e., the driving motor is stopped, as in the preferred embodiments). Therefore, the xe2x80x9cdriving force being switched from the smaller condition to the greater condition includes the following two situations:
1) Driving force is established from zero and increased; and
2) Existing driving force is further increased.
According to a second aspect of the invention, in the aforementioned driving force control unit, the time period required for reaching the driving force value from when the brake pedal is released is controlled to be constant, irrespective of the driving force value in the greater condition.
After the brake pedal is released, the driving force is switched to the driving force value in the greater condition previously determined based on the vehicle inclination angle. However, the driving force is always switched to the preset driving force value for a constant time period, irrespective of the vehicle inclination angle. For this reason, the driver always experiences the constant time period from when the brake pedal is released to the initial motion of the vehicle, notwithstanding the gradient of the road.
According to a third aspect of the invention, a driving force control unit for a vehicle is provided which allows the transmission of the driving force from a driving motor to driving wheels, irrespective of an accelerator pedal being released at a certain or lower vehicle speed, when a transmission is set in a driving range, and which switches the magnitude of the driving force while the accelerator pedal is released at a vehicle speed no more than the certain vehicle speed between a greater condition and a smaller condition based on the brake pedal depression so that the driving force is made lower when the brake pedal is depressed than when the brake pedal is released, wherein the increasing rate of the driving force upon switching from the smaller condition to the greater condition, when the brake pedal is released when the vehicle stops, is increased or decreased based on an inclination angle when the vehicle stops.
According to this invention, the increasing rate of the driving force in the greater condition after the brake pedal is released is made larger at a large vehicle inclination angle and smaller at a small vehicle inclination angle.
As mentioned above, the vehicle inclination angle means the surface gradient of a road on which the vehicle stops. The larger the vehicle inclination angle, the steeper the surface gradient of the road so that a higher rise in the driving force is required to start the vehicle.
Here, the terms xe2x80x9ca certain vehicle speedxe2x80x9d and xe2x80x9cat a certain or lower vehicle speedxe2x80x9d, xe2x80x9ca smaller conditionxe2x80x9d or xe2x80x9cloweringxe2x80x9d of the driving force, and xe2x80x9cthe driving force is switched from the smaller condition to the greater conditionxe2x80x9d are similar to those described before.
According to a fourth aspect of the invention, the vehicle inclination angle is detected based on the braking force generated by the brake pedal depression while the vehicle stops.
The vehicle inclination angle has a constant relation to the braking force that is generated by the brake pedal depression while the vehicle stops. Therefore, the vehicle inclination angle may be determined in terms of the braking force at the time of stopping. This is because the driver depresses the brake pedal strongly on a steep gradient and weakly on a gentle gradient.
As to the vehicle weight (loading weight) on a slope, the heavier the loading weight, the greater the braking force that is required to keep the vehicle stationary on the slope. At the same time, the heavier the loading weight of the vehicle, the greater the driving force (higher rise in the driving force) that is required to start the vehicle. Therefore, if the driving force value is increased/decreased based on the braking force when the vehicle stops, the driving force value is increased/decreased based on the vehicle loading weight.
According to a fifth aspect of the invention, the vehicle inclination angle is determined by the brake pedal releasing rate when the vehicle stops.
Because the vehicle inclination angle has a constant relation to the brake pedal releasing rate when the vehicle stops, the vehicle inclination angle may be determined in terms of the brake pedal releasing rate while the vehicle stops. This is because the driver releases the brake pedal quickly on an up slope and slowly on a down slope.
As to the vehicle weight (loading weight) on an up slope, the heavier the loading weight, the more quickly the brake pedal is released. On a down slope, however, the heavier the loading weight, the more slowly the brake pedal is released. Therefore, if the driving force is increased/decreased based on the brake pedal releasing rate, the driving force value is increased/decreased based on the vehicle loading weight.